Lymphocyte transforming factor (LTF) is a soluble product of activated human lymphocytes that has been shown to permit nonsensitive lymphocytes to respond to antigen by proliferation. The mechanism by which LTF exerts this effect will be studied. In addition the role of this antigen-dependent factor in the proliferation of purified B and T lymphocytes, the generation of antibody-forming cells, and the formation of cytotoxic lymphocytes in vitro will be studied. Investigations will be conducted on the role of this factor in the formation of cytotoxic lymphocytes in the mixed leukocyte reaction and in the development of lymphocytes cytotoxic to human malignant melanoma. The ability of this factor to increase the immune response against melanoma in vivo will be examined. Factors governing the formation and mode of action of suppressor cells in vitro will be studied.